Voltage booster circuits are commonly used and typically comprise a plurality of stages. A basic booster or booster stage uses switches, two capacitors and a two-phase clock to actuate the switches. First, a first capacitor is charged at a voltage Vcc, for example, 5 volts, and then it is discharged into the second capacitor. A second cycle then begins which includes a first step of charging the capacitor at Vcc and then a second step of discharging the first capacitor into the second capacitor (which is already partially charged). Thus, the voltage at the terminals of the second capacitor increases at each clock pulse. Multiple-stage charge pumps are used with complementary switching circuits, and the boosted voltage Vpp then increases to a voltage greater than the supply voltage Vcc.
This principle is commonly used in integrated circuits, especially for electrically programmable memories comprising a floating-gate transistor as a storage element, EEPROM-type memories or EPROM-type memories, depending on whether these memories are electrically erasable or not. The principle may also be used in flash EPROMs when they are block-erasable.
Indeed, in certain memories, the programming voltage Vpp may be supplied by an external supply. However, this then necessitates a specific additional supply pin for the integrated circuit. The additional pins add to the cost of the integrated circuits and it is preferred to avoid them. In certain applications, it is even possible to have no external supply for the programming voltage (for example, in a chip card with a chip having four useful terminals wherein there are no pins or terminals for the programming voltage) and there is no ability to add a terminal. In other applications, it may be that there is no external supply pin at all (for example, in contactless chip cards whose operating power and interfacing are provided by electromagnetic means).
This is why memories in integrated circuit form have been proposed wherein the programming voltage Vpp is produced within the integrated circuit itself from the normal supply voltage Vcc. For reasons of security, it may be preferable, especially in the field of chip cards, to leave the production of the programming voltage under the control of the integrated circuit itself. In this way, fraudulent individuals are not offered the facility of an additional terminal by which this programming voltage could be applied to the circuit and with which they would enjoy an additional means to alter the circuit.
Conventionally, a charge pump or load pump works with an input signal at a switching frequency f. The effects of this switching signal include the generation of switching noise within the voltage booster circuit. This noise increases with the level of the voltage Vpp to be produced and with the number of stages.
The level of output voltage from the step-up circuit, available at a capacitor, is kept at a fixed value by a regulator. This regulator may include a sequence of transistors mounted as diodes so that each transistor sets up a voltage equal to its threshold voltage between its source and its drain. Depending on the technology implemented, the threshold voltage varies, and the number of series-connected transistors is used to define the regulated voltage at the output of the regulator. For example, with transistors having a threshold voltage of about 1 volt, there should be 16 transistors to produce a regulated voltage Vpp on the order of 16 volts.
A charge pump is therefore controlled by an oscillator producing cyclically repetitive pulses that it uses as control signals. Two phenomena then arise; with the first being an amplification of the noise by a resonance phenomenon linked to the control frequency of the charge pump. This noise disturbs the operation of the rest of the integrated circuit. Furthermore, the charge pump itself, with its fixed frequency, could undesirably more easily enter into a state of oscillation induced by a regular external signal.
Furthermore, the electromagnetic radiation induced by the integrated circuit may be undesirable to expose to humans. This is the case for example in portable telephone sets especially GSM (Global System for Mobile Communications) telephone sets which are held close to the user's ear.